Nitrate concentrations in the Morestead borehole, Twyford
2008
This report describes work carried out at Morestead, Twyford as part of a BGS research
project “Nitrate Mass Balance in the Saturated Zone”. The project aimed to evaluate the role
of the diffusive exchange of nitrate between fracture water and porewater in the saturated
zone of the aquifer. The approach adopted attempted to obtain a mass balance for the
catchment to a public supply borehole by comparing nitrogen released from the soil with
nitrogen held in the aquifer and nitrogen pumped to an abstraction borehole.
A new, cored borehole was drilled at Morestead, Twyford in an area of set-aside land on the
margins of a field normally used for cereals. The borehole was completed to 75 m depth in the
former Upper Chalk penetrating about 25 m of the Seaford Chalk Formation and about 50 of
the Lewes Nodular Chalk Formation. The water table lay in the lower part of the Seaford
Chalk.
The core obtained was fractured but most of these fractures appeared to be to be parallel to the
bedding (e.g. along marl seams) and were probably drilling-induced. Some high-angle
fractures with mineralised fracture faces were found in the uppermost 10 m and mineralised
fractures with slickensides (possibly associated with a minor fault) were observed at 31 m
depth, a few metres below the water table at the time of drilling.
A detailed profile of porewater quality was obtained by centrifugation of core samples. The
results indicate that there do not appear to be any zones of unfractured chalk where porewater
has retained pre 1960s concentrations of nitrate. Zones close to major fractures did not show
steep nitrate concentration gradients, suggesting that there were not large differences in
quality between the fracture water and porewater. Porewater concentrations followed a typical
nitrate profile for chalk overlain by arable land with elevated concentrations (up to 18 mg N/l)
in the unsaturated zone and declining concentrations in the saturated zone (up to 9 mg N/l),
except in a 15 m thick zone of the Lewes Nodular Chalk about 25 m below the water table.
Here a number of marl bands appear to result in a zone of slow-moving water with low nitrate
concentration, but elevated concentrations of a range of trace elements often associated with
clay minerals and residence time, particularly Br, Co, Cr, F, Li, Mo, Sb and U.
The results of packer testing of the borehole confirmed that the highest permeabilities were in
the zone close to the water table, with low values at depth, consistent with results from
boreholes in the nearby Candover catchment. Marl seams appear to be much more important
than fractures in controlling groundwater movement to this borehole. Groundwater samples
obtained during packer testing were all of similar composition and were interpreted as being
drilling water which had not been fully flushed from the borehole before the test. The
modelling step of the work was therefore not attempted as this required nitrate concentrations
in the mobile water in the aquifer.
The unsaturated zone porewater profile indicates nitrate concentrations mainly at and above
the current drinking-water standard of 50 mg/l nitrate (11.3 mg N/l). Annual mean nitrate
concentrations at Twyford are rising towards 50 mg/l with seasonal peaks exceeding these
concentrations. If this is representative of local conditions, and given the significant
proportion of similar arable land in the immediate catchment of the Twyford boreholes then
groundwater concentrations are likely to continue rising under the present landuse and
agricultural regime. Moreover much of the cultivated land is located in the upper part of the
catchment and the nitrate is likely to still be present in the unsaturated and saturated
groundwater flow path. Even close to the Twyford borehole, the ‘improved grassland’ may
have significant applications of inorganic fertiliser and organic manure. Present conditions do
not therefore suggest any immediate reduction in the upward groundwater nitrate trend.
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